The homogeneous nucleation of silicon from the Si-H-Cl system was studied theoretically for a wide range of compositions and temperatures. Contour plots of the conversion ratio and the chemical potential to nucleate are displayed. Nucleation rates were calculated using a modified classical nucleation model (Girshick et al. (1990) Aerosol Sci. Technol. 13, 465) which agrees well with reported data on the onset of nucleation for the SiH4/H-2 and SiH2Cl2/H-2 systems. It is important to use a correctly defined supersaturation and a temperature-dependent surface tension. Theoretically, the nucleation time lag is found to be substantial in the vicinity of the nucleation onset and at high silane concentrations. The decomposition kinetics of the silane becomes important in systems with a very fast temperature rise, for example in a laser-heated reactor.